Fractionating tower



p 23, 1958 H. J. HIBSHMAN ETAL 2,853,281

FRACTIONATING TOWER 3 Sheets-Sheet 2 Filed March 26, 1952 Robert J.Wimmer, Jr. Henry J Hlbshmun Inventors Stephen H. Dole Y W JJ" AttorneySept. 23, 1958 H. J. HIBSHMAN El-AL I 3,

FRACTIONATING TOWER .Qobert. J 251mm 51'.

v Stephan HIDdle.

Unitd States Patent FRACTIONATING TOWER Henry J. Hibshman, Plainfield,Stephen H. Dole, Westfield, and Robert J. Wimmer, Jr., Roselle Park, N.J., assignors to Esso Research and Engineering Company, a corporation ofDelaware Application March 26, 1952, Serial No. 278,728 2 Claims. (Cl.261-114) The present invention is concerned with an improved apparatusfor operating a countercurrent-vapor-liquid treating zone. The inventionis more particularly concerned with an improved fractionation zone andis especially directed to an apparatus for contacting upflowing vaporand downflowing liquid utilizing contacting vapor trays or theirequivalent. In accordance with the present invention, the capacity of atray and the entire treating zone is markedly increased by providing anapparatus for efficiently and readily contacting the countercurrentflowing phases. In accordance with the present invention, a directionalvapor stream is produced on the respective trays which facilitates thepassage of the downflowing liquid across the tray, thus reducing liquidholdup on the tray. The directional stream'issecured by a contactingtray-which is characterized by containing tab openings.

It is Well known in the art to carry out many chemical reactions andseparations wherein vapor and liquid are contacted in a countercurrentzone, such as in a vapor liquid fractionation 'zone. Normally the liquidpasses from one zone to a lower zone by means of downcomers or theirequivalent while the vapors pass upwardly'from zone to zone throughchimneys in the tray, around various types of bell caps or theirequivalent into the liquid phase disposed on top of the tray. The liquidphase flows across the tray and over weirs on the respective trays intodowncomers and onto the tray in the zone below. The height of the liquidphase 'on the tray is determined by the height of the weir. Thedowncomer from the zone above must of necessity extend below the top ofthe liquid phase on the lower tray in order that vapor will not pass upthrough the downcomer instead of through the bubble caps. In liquid-gascontacting operations of this character, the capacity of the tray andconsequently the tower is determined to a large extent by the degree ofefiiciency with which the downfiowing liquid flows across the tray andinto the downcomer. Thus, aside from limitations of auxiliary equipment,such as furnaces, feed pumps, and condensers, the capacity of afractionation tower is determined by several factors. Basically, theseare limitations to passage of liquid down and vapor up the tower in sucha manner that efiicient contacting is achieved.

The requirement of eflicient contacting means that the limitation may beone of too rapid or free passage of one or more of the phases throughthe tower, as Well as .restrictions to flow of the phases. Tray dumping,liquid running down through bubble "cap chimneys, is .an example of toofree flow of .liquid. Downcomerfilling with backup of liquid on'thetray'is the opposite" type of ice limitation, resulting in poorefiiciency because of excessive entrainment and ultimately 'in towerflooding. .A similarhigh entrainmentresult .is produced by excessivevapor rates. In a typical bubble cap tower, each of these limitationspredominates .over a dilferent vapor rate range. I

One operating disadvantagecornprises excessive liquid holdup which, inthe absence of a downcomer limitation and obstructions -.on the tray, isdetermined by the linear velocity at which liquid is able .to passacross the tray. For a given liquid velocity across the tray the liquidholdup is directlyproportional to the volume of liquid flowing acrossthe tray :in a unit of time. Since the liquid on the tray is aerated bythe vapor, the volume occupied by the liquid is .a function of thevelocity of'the vapor in the tower and the amount of liquid holdup onthe tray. At normal tray spacingsa tower will ultimately be limited incapacity .by the liquid flow approaching the tray above, resulting .inexcessive entrainment. .Accordingly, higher capacities can be reached ifthe liquid holdup is reduced. Reduced holdup is accomplished by thepresent invention which directs the upflowing vapor stream through thetrays in such a manner as to push liquid across the tray at a fasterrate. a

In accordance with the presentinvention, liquid holdup on the respectivetrays is prevented .by providing concurrent fiow of upflowing vapor andliquid across the tray. In other words, a directional flow of vapors isprovided in order to increase the velocity of the liquid flowing acrossthe trays, thus minimizing liquid holdup. .It is an important object ofthe invention to provide such directional flow by employing platesprovided with a large number of punched tabs distributed over asubstantial portion of their area, said tabs being punched so as todirect upflowing vapor in a direction toward the conduit associated witheach plate, .said tabs having tab angles which decrease progressively inthe direction along the plate toward a conduit such as a downcomer.

The present invention may be readily understood'by reference to thedrawings illustrating embodiments of the same. Figure 1 illustrates atypical bubble cap tray fractionation zone, While Figure 2 illustrates afractionation zone employing tabs of the present invention. Figure 3 isa top view of the tab tray of the present invention, while Figures 4A to4B illustrate various types of tabs which may be utilized. Figure '5 isa side view of the tab and illustrates the tab angle.

Referring specifically to Figure 1, the numeral 10 designates a bubblecap tray column, contacting vessel. The vessel 10 is conventionallyprovided with a series of vertically spaced, transverse, perforatedplate elements 11, forming a vertical series of superimposed, contactchambers or zones. These chambers or zones are in communication with oneanother by way of the passageways 12 formed by the plate perforations,.and downcomers 13 disposed at alternate sides of the vessel from plate.to plate. The downcomers extend from the surface of one platedownwardly into vertically spaced relation to the surface of the platenext below. As shown in Figure 1, the passageways 12 through the plateare each provided with bubble cap elements 14. In addition, :each plate11 is provided with a weir member 15, at the entrance to the downcomer13, extending upwardly from the plate surface to a level above the lowerend of the downcomer from the plate next above. The vessel is also 3provided with a feed line 16. An outlet from the vessel for gaseousmaterials is provided as by conduit 17. At the lower end of the vesselis an outlet 19 for heavier materials. In operation, it is to beunderstood that any number of trays may be utilized. In accordance withthe present invention, for the purpose of description, it is assumedthat a vaporous feed is introduced at an intermediate point offractionating zone by means of line 16. Temperatures and pressures areadjusted so that vapors flow upwardly in zone 10 through the chimneys 12and around bubble caps 14 into a liquid maintained on the top of theplate, the height of which is determined by weir 15. The liquid phase onthe plate comprises dissolved or condensed constituents of the vaporousfeed introduced by means of line 16. The liquid phase which flows downthrough zone 10 over each plate and weir in succession is known asreflux. The reflux is normally initiated by condensing a portion of thevapors withdrawn through line 17 and introducing the liquid condensateinto the top of zone 10 through line 20. Vapors for the section of zone10 below feed line 16 are obtained by vaporizing a portion of the heavyproduct withdrawn from line 19 and introducing the said vapors throughline 21.

Figure 2 is identical to Figure 1 except that the bell caps 14 arereplaced by tabs 42.

Referring specifically to Figure 3, liquid flows downwardly onto plate40 through downcomer 41. The liquid flows across the tabs 42 on theplate through which upflowing vapors pass and contact the cross-flowingliquid. The liquid passes across weir 43 into downcomer 44 in which theliquid passes to the zone below. 7

Figures 4A to 4E illustrate various types of tabs A, B, C, D and E,which may be utilized. Figure 5 shows 'a tray 50, a tab 51 and the tabangle.

Another advantage of the present invention is that reduced tray inletliquid heads are secured. In a normal bubble cap tray operation there isa tendency for liquid to build up to a higher level at the inlet side ofthe tray than anywhere else on the tray. It has been found that the useof tabs in place of bubble caps produces the opposite effect. In thiscase, the inlet head is lower than anywhere else on the tray.

The design of the tabs is not critical. Hemispherical baffles open onone side towards the downcomer, square or rectangular box-type bafflesopen towards the downcomer; elbow-type baffles and similar devices maybe used.

A simple and effective arrangement may be fabricated by punching platetrays to provide the required openings, leaving inclined flaps of metaladjacent each opening to direct vapors toward the downcomer.

The bafiles may be equipped with slots on their open ends and they mayhave holes or clearance with the tray floor on the sides other than thatfacing the downcomer. However, they must produce a directional vaporstream leaving the tray in the direction of the downcomer.

In accordance with the present invention the tabs on the tray arepunched or fabricated so that the tab angle is within a critical range.The tab angle is the angle between the plane of the tab and the plane ofthe tray. It has been found that the tab angle and distribution of tabsand tab angles across the tray are highly critical and that satisfactoryoperation can be achieved only if proper consideration is given to thesefactors. For instance, if a uniform tab angle is utilized with respectto the tabs across the tray, it is essential that the tab angle bebetween about 12 and 25, preferably between and 20. At tab angles lessthan about 12, the vapor handling capacity is no. higher than withconventional bubble cap trays. At tab angles higher than about the traysdump excessively, thus limiting the range of operability. This isillustrated by the following example.

Example 1 Various operations were conducted using various tab angles,the results of which are shown in the following table H3) Liquid TabAngle Rate, Vapor Rate Vapor Rate Operability G. P. H at FloodatIncipient Range, Sq. Ft Point, Dumping, Ft./Sec. FtJSec. FtJSec.

800 3. 95 2. 37 1. 58 800 7. 43 4.33 3. 10 800 10. 14 5. 94 4. 20 80010. 65 6. 36 4. 29 800 11. 12 7. 18 3. 94 800 10. 90 8. 02 2. 88 80010.06 8. 61 1. 45 800 8. 73 4. 66 4. 07 305 3. 95 2. 25 l. 70 305 7. 004. 07 3. 02 305 9. 46 5. 56 3. 90 305 9. 89 5. 94 3.95 305 10. 10 6. 363. 74 305 9.16 6. 28 2.88 30 305 6. 79 5. 62 1. 27 Bubble Cap Trey 3059. 46

1 Data taken from 10-ft. diameter experimental half tower using air andwater. Tray had 344 2-in. tabs on 3.5-in. centers, staggered pattern.

From the above it is apparent that if a satisfactory operating range isto be secured, the tab angle should be between 12 and 25 when utilizinga uniform tab angle across the tray.

A preferred mode of operation is to employ tab angles of decreasingvalue across the tray in the direction of liquid flow. This isillustrated by the following example:

Example 11 A number of operations were carried out utilizing a 10-ft.diameter tower. With the conventional 20 angle directional jet trayhaving 344 tabs 2 inches wide arranged in staggered or triangular pitchwith 3 /2 inch centers, the l0-ft. diameter tray would only handle30,000

cubic feet of vapor per minute when 525 gallons per about 15 and 20.

minute of liquid was flowing across the tray. When the tab angle wasraised to 45 on the inlet /3 of the tray and decreased to 10 on theoutlet side of the tray, the vapor handling capacity of the tray wasincreased to 35,000 cubic feet per minute. This is an increase of 17%.

Thus, when utilizing tabs having angles which decrease in the directionof liquid flow, the angles of the tabs are not within the 12 to 25critical limit discussed heretofore. Under these conditions the first /3of the tabs should have angles in the range from about 30 to 60, themiddle third of the tabs should preferably have angles in the range fromabout 12 to 24, while the last /3 of the tabs in the direction of liquidflow should have angles in the range from about 5 to 15. Thus, if allthe tabs on a directional jet tray are bent to the same angle, thecritical angle has been found to be between Under these conditions,trays having angles greater than 25 or less than about 12 aresignificantly inferior.

However, the combination employing graduated tab angles produce betterresults with respect to operability range and maximum vapor handlingcapacity. Under these conditions the tab angles should be as specifiedabove. It is of note that other combinations were tried but found to beinferior. For example, raising all the tabs to but reducing vapor flowon the outlet /3 side of the tray by blanking every other row of tabsgave poor performance. Lowering of the inlet /3 of the tabs to 45 toaffect an increase in the rate of liquid flow across the tray producedinferior results. Poor results were also secured when the center of thetray was blanked 011. Other inferior results were secured when everyother tab was lowered on the inlet side of the tray from 45 to 20.

What is claimed is:

1. Apparatus adapted for contacting upflowing vapor and downflowingliquid comprising a vertical tower containing a plurality ofhorizontally disposed vertically spaced plates extending substantiallyacross the tower, a vertically positioned conduit extending through eachof said plates terminating above and below each plate is spaced relationto successive plates, in which said plates are provided with a largenumber of punched tabs distributed over a substantial portion of theirarea, said tabs being punched so as to direct upflowing vapor in adirection toward the conduit associated with each plate, said tabshaving tab angles which progressively decrease in the direction alongsaid plate toward said conduit.

2; Apparatus as defined by claim 1 wherein said tabs furthest away fromsaid conduits have tab angles in the range from about 30 to 60 whereinsaid tabs in the central area of said plate have angles in the rangefrom about 12 to 24 and wherein said tabs nearest said conduits haveangles in the range from about 5 to 15.

References Cited in the file of this patent UNITED STATES PATENTS

